1. Field of the Invention
The present invention relates to piezoelectric sensors and energy harvesters.
2. Description of the Related Art
Piezoelectric sensors have been used in energy harvesters (also called generators) and other applications. Typical types of sensors used in such applications include bimorph-type and multi-layered devices.
Bimorph sensors produce a voltage in response to the application of a bending force. These sensors have been used in energy harvesters (see Olney, U.S. Pat. No. 5,934,882). However, in such devices the energy that can be harvested is limited to the applied force. Additionally, bending causes fatigue of the piezoelectric material, thus shortening its lifetime.
Multi-layered piezoelectric stacks are also used in energy harvesters (see Olney). In such devices a direct force, orthogonal to the major surfaces of the piezoelectric stacks, is applied to compress the stack.
Again, the energy that can be harvested is limited to the applied force.
U.S. Pat no. 5,729,077 discloses a structure for an actuator which has amplifiers on opposing major surfaces of a piezoelectric substrate. Piezoelectric actuators and sensors have similar structures, with actuators used to generate a displacement force from an applied voltage, rather than to generate a voltage from an applied force as in a sensor. The structure disclosed in the ""077 patent can be used in a sensor embodiment, where force applied to the amplifiers causes a tensile stress in the piezoelectric substrate.
The ""077 patent teaches interdigitated input and output electrodes within a multilayered piezoelectric body. The electrodes are not continuous along the length of the device, but rather are connected to conductors at one edge and terminate internally without connecting to the opposite edge.
The multilayered structure reduces the voltage required to produce a given displacement because the relatively short distance between successive electrodes produces a relatively high electric field for a given voltage differential between the electrodes. However, in the sensor embodiment, the multilayered structure has a relatively low voltage output per unit of applied force and a relatively higher internal capacitance, and is more costly to construct. Moreover, the device has a relatively low tensile stress tolerance.
The new energy harvester comprises at least one sensor having a piezoelectric plate with its polarization orthogonal to its major surfaces, throughout the length of the plate. When a tensile force is applied to either stretch or compress the plate along its length, the piezoelectric material produces an electric charge via the direct d31 piezoelectric effect.
The sensor has a force amplifier that translates a perpendicular force into an amplified tensile force along the length of the plate. Due to the magnification of the applied force, the sensor produces a high energy and voltage output in response to relatively small forces.
Additionally, the sensor has electrodes only on the opposite major surfaces of the piezoelectric plate, covering the entire surfaces. The piezoelectric plate""s monolithic structure and electrode configuration contribute to the sensor""s high output voltage per applied force. Additionally, the sensor has a reduced internal capacitance, is relatively low cost, and can withstand a high tensile stress.
One embodiment of the new harvester employs a lever arm that reciprocates between a pair of the piezoelectric sensors to place alternating compressive tensile stresses on the sensors, thus converting mechanical work into electrical energy. This device can be tuned to operate at a particular frequency. With a mass placed on the lever arm, giving the mass an alternating motion produces the required reciprocation of the lever arm. In another embodiment a mass held between two sensors, which similarly imparts alternating compressive stresses on the sensors as the mass vibrates.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.